1. Field of the Invention
The present invention relates generally to wireless networks and more specifically to a system and method of erasing location information on a SIM card in a mobile telephone.
2. Discussion of Related Art
In a Global System for Mobile Communication (GSM) system, mobile phones use subscriber identifier modules (SIMs) associated with the mobile phone to store data. The SIM may be a check card or a plastic piece about 1 cm square that may be plugged into the mobile phone. The SIM enables a user to insert the SIM into any mobile phone and use that phone with the user's account. Typically, the type of data stored includes the personal identification number for the user, lists of the optional functionality of the SIM, redial numbers, charges for time increments of phone use, subscriber data, secure data transmission parameters, roaming data and public land mobile network (PLMN) data. The present disclosure is directed to the use of the roaming data with its associated location information on the SIM card.
The SIM card is illustrated in FIG. 1. The wireless device 10 may be a GSM mobile phone or any other wireless device such as a personal digital assistance or portable computer that uses a SIM card or a similar permanent or detachable data card the functions like a SIM card to store roaming data as described herein. The SIM card can be either an ID-1 SIM 12 or a Plug-in SIM 14. The ID-1 SIM is a check card having a microchip or a plug-in card 14 that is about 1 cm square. These descriptions and dimension are not meant to be limited but only serve to generally describe a SIM card. Most SIM cards have the same ability to be inserted into a wireless device 10 such as a mobile phone and operate to provide parameters and data to operate the wireless device 10. The wireless device 10 communicates through an air interface with a base station 16 and an associated wireless network 18. The details of such a network are known to those of skill in the art, therefore, they are not discussed more herein.
The PLMN data include the network color code (NCC), which is a three-bit-long code that identifies the PLMN. The NCC is part of the BSIC and is broadcast in the synchronization channel. Other PLMN parameters include the mobile country code (MCC), which is a three-digit identifier that uniquely identifies a country (not a PLMN) and the mobile network code (MNC) of the home PLMN.
The roaming data includes location updating status parameters for determining whether to update the location data, location area information (LAI), and the NCC of preferred PLMNs. The roaming data also includes a list of preferred PLMNs or service providers that the mobile phone should select if more than one service provider is available and the home PLMN is not found.
When a GSM mobile phone is powered on, the mobile phone needs to select a service provider. The current PLMN service provider is stored on the SIM card as part of the location information. The mobile phone checks the stored service provider in the location information upon power up before looking for the home service provider or another preferred service provider. If the mobile device finds a service provider using the PLMN service provider stored in the location information on the SIM, it will use that service provider.
The approach of first searching at power up for a service provider using the service provider stored on the SIM card causes several problems. First, suppose the user's previous use of the mobile phone was with a roaming service provider outside of the home-use area and the next power up by the user is in his or her home PLMN. In this scenario, if the roaming service provider also provides service in the home service area of the mobile user, the mobile phone may register with the roaming service provider when it should register with the user's home service provider.
Second, as discussed above, part of the roaming data includes the NCCs of preferred PLMNs with which the mobile phone should register if the home PLMN is not available. If the user is roaming outside his or her home service area, and another more preferred service provider is listed in the roaming data, the mobile phone may still attain service from the service provider listed as the current service provider in the location information although that service provider is lower down or not even on the preferred service provider list.
Accordingly, for the foregoing reasons, there are drawbacks to the present system and method of searching for a service provider when a GSM mobile phone is powered up.